1. Field of the Invention
The present invention relates generally to an optical information recording medium that allows information to be recorded and reproduced at a high density and a high speed through an optical means such as, for example, laser beam irradiation.
2. Related Background Art
Optical information recording media such as, for instance, magneto-optic recording media and recording media of a phase-change type, have been known as media that allow a large quantity of information to be recorded and reproduced at a high speed. In these optical information recording media, the change in optical characteristics of a recording material that is caused when the recording material is irradiated locally with a laser beam is used for recording marks. These optical information recording media have great advantages in that they permit random access as needed and have excellent portability. Hence, recently, their significance has been increasing more and more. The demand for them is increasing in various fields including a medical field, an academic field, etc. and for various purposes including, for example, recording and storing personal data or image information with computers and substituting for home video tapes. Nowadays, these optical information recording media have been required to achieve further increases in capacity, density, and speed due to the improvements in performance of applications and quality of image information.
The conventional optical information recording media include rewritable recording media in which information can be rewritten multiple times and write-once recording media in which information can be written only once. Generally, the write-once recording media often have a smaller number of layers than that included in the rewritable recording media. Accordingly, the write-once recording media can be manufactured easily at lower cost. In addition, they are convenient for writing data that a user wants to prevent from being destroyed since they are not rewritable. Furthermore, they have a long storage life and high reliability. Thus, they are expected to be in great demand for archival use. Hence, conceivably, the widespread use of high-density, rewritable recording media results in further increase in demand for high-density, write-once recording media.
Conventionally, a recording material whose main component is a material containing, for instance, Te and O (hereinafter also referred to as “Te—O”) has been proposed as an example of the recording material for the write-once type. It has been disclosed that the use of the recording material whose main component is Te—O (Te—TeO2, which is a mixture of Te and TeO2) allows a large signal amplitude and considerably high reliability to be obtained (see, for instance, T. Ohta, K. Kotera, K. Kimura, N. Akahira, and M. Takenaga, “New write-once media based on Te—TeO2 for optical disks”, Proceedings of SPIE, Vol. 695 (1986), pp. 2–9). When a recording layer formed of such a recording material is used alone, a large signal amplitude can be obtained in the case of a so-called Lo-to-Hi configuration in which the recording layer has a low reflectance in the as-deposited state (i.e. the state immediately after the formation thereof, hereinafter also referred to as an “as-depo state”) and a high reflectance after information is recorded therein. On the other hand, it has been disclosed that when the recording layer is interposed between layers formed of dielectrics, a large signal amplitude can be obtained even in the case of a so-called Hi-to-Lo configuration in which the recording layer has a high reflectance in the as-depo state and a low reflectance after information is recorded therein (see, for instance, JP2002-133712A). Generally, the optical information recording media of the rewritable type have the Hi-to-Lo configuration. Hence, it is preferable that in the write-once recording media, the change in reflectance occurs in the same manner as in the rewritable recording media, since this allows information to be recorded more easily in write-once recording media using a drive for the rewritable recording media.
Furthermore, it is intended to increase the capacity of recording media in which a Te—O-based recording material is used by using a plurality of information layers stacked together (see, for instance, K. Nishiuchi, H. Kitamura, N. Yamada, and N. Akahira, “Dual-Layer Optical Disk with Te—O—Pd Phase-Change Film”, Japanese Journal of Applied Physics, Vol. 37 (1998), pp. 2163–2167). This reference discloses the technique of recording and reproducing information with respect to two information layers through laser beam irradiation alone that is carried out from one side, wherein the two information layers are stacked with an intermediate layer being interposed therebetween.
However, in order to achieve further increase in capacity of the optical information recording media, information is written under conditions for achieving higher density, for instance, using a laser beam with a shorter wavelength within a bluish-purple wavelength region and an objective lens having a numerical aperture (NA) of at least 0.80. This raises a problem that a favorable jitter value cannot be obtained readily. Furthermore, in media including a plurality of information layers stacked together, when information is recorded or reproduced with respect to each information layer through the laser beam irradiation that is carried out from one side under the conditions for achieving higher density, the information layer located on the laser beam incident side must have a sufficiently high transmittance with respect to the laser beam and favorable recording characteristics. However, when the transmittance of the information layer is kept high; for instance, at least 50%, it is difficult to obtain a favorable jitter value, which is a problem. In the conventional optical information recording media described above, no consideration has been taken to obtain a favorable jitter value when a plurality of information layers are stacked together or when information is recorded under the conditions for achieving higher density. For instance, it has not been considered to provide the media with means for obtaining higher signal quality when the in-plane recording density in the recording film is increased using a laser beam with a wavelength within the bluish-purple wavelength region and an objective lens having a NA of at least 0.80. Moreover, in the case of media with a multilayer configuration in which a plurality of information layers are stacked together, it also has not been considered to improve the transmittance of the information layer located closest to the laser beam incident side of the plurality of information layers while maintaining high signal quality.
In order to solve the problems described above, it is possible to make an effective optical design by using an optical multiple interference effect that is obtained by providing the information layer with a more complicated configuration. However, this is not preferable since, for example, optical information recording media of the write-once type are required to include information layers that each have a simple configuration and to be manufactured at low cost.